Corrosion and wear resistant iron alloy

ABSTRACT

Erosion and wear resistant iron alloy, comprising: Cr 20.0-36.0% by weight, B 2.5-5.0% by weight, Mn 0.5-3.2% by weight, Si 0.05-0.6% by weight, Mo 0.3-2.5% by weight, V 0.05-0.3% by weight, Nb 0.03-0.3% by weight, P 0.5% by weight or less, C 0.05-0.3% by weight, and a trace of unavoidable impurities, which confers superior corrosion and wear resistance upon abraded or corroded portions by coating or molding.

This application is a division of application Ser. No. 08/563,689, filedon Nov. 28, 1995, now U.S. Pat. No. 5,635,255.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an iron alloy composition superior incorrosion and wear resistance. More particularly, the present inventionrelates to an iron alloy composition which confers corrosion and wearresistance upon abraded or corroded portions by coating or molding.Also, the present invention is concerned with a method for preparingcorrosion and wear resistant members using the same.

2. Description of the Prior Art

In the past, in order to bestow wear resistance on iron materialsvarious techniques including carbonizing, nitrification, inductioncuring, chrome plating and ceramic coating were employed. However, theiron materials treated by such techniques are not sufficientlysatisfying ones because of low surface hardness, brittleness or lack ofadhesion strength.

Various attempts have been made to circumvent the problems but there arestill some disadvantages. For example, low temperature sulfurizingtreatment, such as caubet treatment, could bring an improvement intowear resistance but, since the treated object turned out to be poor insurface pressure resistance, the treatment is not suitable for low speedand high surface pressure conditions. Electroless nickel plating issuperior to other preexisting techniques in hardness and wear resistancebut it has difficulty in practical use because of the many restrictionsin treatment condition and it has limit in thickening surface layer.

These various problems have forced many researchers into developingtungsten carbide sintered alloy, which is currently the most widelyused. However, it is difficult to apply the tungsten carbide sinteredalloy in various fields. In practice, it is applied only for few fieldsincluding, for example, mold materials and processing tools. The reasonis that mold is required for its manufacturing. In addition, there aremany problems in impact resistance as well as time and cost.

Recently, thermal spray techniques which use Ni-based alloy or Mo, W, Zror Nb-based alloy have been expected to overcome the above problems. Ofthe techniques vitrification, by which metal is made to be vitriform,has been in the limelight because it was expected basically to surmountthe aforementioned problems basically.

In 1960, Duwez introduced the possibility of manufacturing vitriformalloy by quenching molten metal into the world, which was patented inU.S. Pat. No. 3,297,436 in 1967. Since then, many related patents havebeen yielded. Vitrification of Fe alloy was disclosed by H. S. Chen inU.S. Pat. No. 3,856,513 and a more detailed alloy composition wassuggested in U.S. Pat. No. 3,986,867 to Masumoto. However, a significantdisadvantage of these alloys is that, when they are produced in bulkphase, brittleness occurs because of the redundancy of phosphorous andcarbon. These materials show limited vitriform property and are short ofthermal resistance. Lack of thermal resistance causes them torecrystallize from the metastable vitriform phase, resulting in a lossof vitric characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above-mentionedproblems encountered in prior arts and to provide an iron alloy which issuperior in erosion and wear resistance and which shows a stable andstrong toughness structure.

It is another object of the present invention to provide a method forpreparing erosion and wear resistant members using the iron alloy.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing the change of free energy with regard totemperature.

DETAILED DESCRIPTION OF THE INVENTION

In general, it is known that metal and its alloy have crystallinestructures in the solid state. Besides, it is inferred that melted metalor alloy is already in quasi-crystalline state. It is possible tovitrify a particular alloy composition by restraining long-range orderedstructure from being formed therein during rapid solidification fromliquid or solidification through depositing or plating, or by implantingenough ions to oversaturate essential elements, which results indifficulty forming the long-range ordered structure.

Solid vitriform is stable when metastable phase with a high free energy(ΔG) is above the shear instability. As for phase stability ofvitriform, where the enthalpy of mixture is far less than 0, that is, ΔHmix<0. the vitriform phase makes a condition in which it becomesstable. In order that the enthalpy of the mixture has a large absolutevalue with negative sign while the entropy of the mixture has a largepositive value, for example. A-B type binary alloy composition shouldsatisfy the condition of E_(AB) >>(E_(AA) +E_(BB))/2. Within suchcomposition range metastable intermetallic compounds form uponsolidification or subsequent thermal treatment. As temperature islowered in a peculiar phenomenon, inverse melting, appears. In otherwords, the lowering of temperature causes the transformation of thesolid state into a liquid state and the phase resulting from the inversemelting is of vitriform structure.

Referring to FIG. 1, there is a curve that shows the change of freeenergy with regard to the temperature. As shown in this figure, suchtransformation is a thermodynamically possible process.

In order to satisfy such spontaneous vitrification, a composition isrequired in which the high temperature (just below melting point,T>0.8T_(m)) phase should be present as a single phase and should bemaintained as a quasi-equilibrium phase even at ordinary temperatures.In this state, an external driving force allows the metastable phase totransform into a vitriform phase. This vitriform transformation isattributed to the fact that the nucleation of stable intermetalliccompound is difficult at relatively low temperatures and a distortionattributable to grid defect makes spinodal decomposition difficult.

Methods for obtaining vitriform transformation from such metastablephase include mechanical pulverization, polishing, energy scanning, coldforming and quenching through thermal treatment. The present inventorshave recognized that a metastable phase, after being established, wouldbe possible to vitrify even with abrasive stress and researched formaterials which could be vitrified with shear stress resulting fromvertical load and abrasion. In this regard, the free energy of theabove-mentioned intermetallic compound-forming alloy can be heightenedby increasing the number of grid defects in metastable solid solution.On the basis of this, an iron alloy composition has been developed thatcan be spontaneously vitrified from metastable crystalline state to asuper-cooled liquid state through inverse melting.

In the invention, whether the wear-resistant material is coated on apart of article or is used as the whole material of article, it isinitially in a metastable crystalline state and is subjected tostress-induced vitrification by high surface pressure during processing.Besides, even though the material of the present invention is not invitriform state, it is a stainless steel composition which is superiorin corrosion resistance and toughness by virtue of Cr, Al and Mo alloyelements and in wear resistance by virtue of B element.

The present invention provides a method for producing an iron alloywhose outermost surface is maintained by stress induction when it issubjected to vitrification and which shows corrosion and wear resistanceeven in a non-vitriform state, and the application techniques thereof.One of the problems generated in conventional wear-resistant iron alloyis that, because the improvement of wear resistance by high hardness isaccompanied by extreme degradation of toughness, resulting in lack ofimpact resistance, there is a limit in applying the iron alloy for anarticle of bulk state. In order to solve the problem, the method of thepresent invention comprises the formation of a metastable solid solutionto reinforce the solid solution, thereby maintaining wear resistance.Particularly, the outermost surface in abrasion-worn portion of the ironalloy is vitrified by stress-induced transformation, which results inbringing superior physical properties into surface. Accordingly, whilesupporting the high hardness surface layer the base of the coating layershowing the metastable phase absorbs impulsive load. Meanwhile, thesurface is responsible for vertical load or shear stress. As a result,toughness and wear resistance are provided.

Preexisting ceramic materials are known to have high hardness andsuperior wear resistance but, in practice, their uses are extremelylimited owing to their brittleness. To overcome this, aslope-functioning material is conventionally employed to relieve thehardness difference from the base layer and to improve adhesion thereto.However, in the present invention, the coating layer itself is of ametastable phase and has a hardness of HV 500 to 700 by virtue of solidsolution reinforcement and, when the outermost surface is subjected tovitrification, it has a hardness of HV 1200 or higher, showing a naturalhardness slope. In addition, while the surface layer plays a role oflowering the friction coefficient in order not to generate seriousabrasion scars, such as scoring and wipering, the base layer uniformlydisperses the surface pressure to show high adhesion strength. Theseproperties of the present invention complement the disadvantages of thepreexisting ceramic materials.

Basically, the wear resistant iron alloy of the present invention isformed into bulk and powder state. In the case of powder, there may beused (1) pulverization after induction melting in vacuum, (2)pulverization after remelting at least once in vacuum arc furnace, (3)quench solidification using melt spinning or atomization, or (4)mechanical alloying of raw material powder. After being subjected tosurface treatment, the powder obtained by such methods can be coated,giving the excellent corrosion and wear resistance.

For bulk state, secondary materials made by conventional casting methodsusing centrifugal casting or moulds are cut into sizes suitable for useor subjected to machining and, then, only surfaces of desired portionsare rapidly heated by use of high frequency induction heating, electricbeam irradiating or laser surface treatment, so as to construct ametastable phase on the surface. Meanwhile, it is difficult to obtainthe metastable phase from the casting state itself. In accordance withthe present invention, a rapidly solidified layer on a surface can beobtained with a rotating centrifugal castor or molding castor byinserting molten metal in a pipe type mould. This is what results fromthe application of effect of rapid solidification, an applied process inpreparing powder, for bulk materials. Cooling rate can be controlled bywinding a cooling line round a mould. Likewise, the same effect can beachieved in mold casting.

In accordance with an aspect of the present invention, there is providedan iron alloy, comprising Cr 20.0-36.0% by weight. B 2.5-5.0% by weight,Mn 0.5-3.2% by weight. Si 0.05-0.6% by weight. Mo 0.3-2.5% by weight, V0.05-0.3% by weight. Nb 0.03-0.3% by weight, P 0.5% by weight or less, C0.05-0.3% by weight, and a trace of unavoidable impurities with apreference of Cr 26.0-28.0% by weight. B 3.2-3.6% by weight, Mn 0.3-1.0%by weight, Si 0.2-0.5% by weight, V 0.1-0.3% by weight, Nb 0.05-0.2% byweight and the sum of P and C 0.5% or less.

In accordance with another aspect of the present invention, there isprovided a method for preparing wear-resistant members from powder ofthe iron alloy material, comprising

(1) coating by plasma, arc or flame thermal spray;

(2) sputtering with said powder serving as a target;

(3) ion plating or evaporation with said powder serving as a source;

(4) thermal spraying or cladding-by-welding of a cored wire includingsaid powder on the surface of various base material; or

(5) slurry coating a member with said powder, thermal coating it ininduction or electric furnace, atmosphere furnace or vacuum furnace, andcutting with press and forming.

In accordance with a further object of the present invention, there isprovided a method for preparing wear-resistant members from the bulkiron alloy by centrifugal or mold casting of molten metal, comprising asurface-treatment technique by which the surface of said member can beheated rapidly up to just below the melting point, such as

(1) high frequency induction hardening;

(2) laser hardening; or

(3) electrical beam irradiating.

A detailed description will be given of the iron alloy composition.

Together with Fe, Cr forms into homogeneous solid solution and is themost important element to improve strength and corrosion resistance. Inthe present invention, Cr is added at an amount of at least 20% byweight. This amount of Fe is necessary to restrain the long-rangeordered structure at just below the melting point, thereby allowing themetastable phase, the necessary condition for vitrification at ordinarytemperatures, to be formed. If Fe is added at an amount less than 20% byweight, α-ferrite phase may be rapidly formed in liquid state, which iaa type of partitionless transformation not requiring the partition ofcomposition. Thus, it becomes difficult to restrain the transformationinto α-ferrite phase, a crystalline phase, which gives difficulty toforming the vitriform phase. On the other hand, if Fe is added at anamount more than 36.0% by weight, a driving force may occur which existsas a stable mixture phase of α-ferrite and Cr, deleteriously affectingthe formation of vitriform phase.

The addition of semi-metal element is indispensable to obtain thevitriform phase. In this regard, B plays an important role in thepresent alloy composition. B lowers the melting point and reacts with Feand Cr to form compounds, Fe₂ B and Cr₂ B, which form homogeneous solidsolution additionally and show metastable phase at ordinarytemperatures. In addition, Fe₂ B+Cr₂ B forms an absolute solid solutionat high temperatures which is helpful in the formation of the metastablephase at ordinary temperatures, the base for solid phase vitriformtransformation, thereby promoting vitrification of the alloy. B ispreferably added at an amount ranging from 2.5 to 5% by weight. Forexample, if too little B is used, the amount of Fe₂ B+Cr₂ B, importantfor formation of metastable phase, is reduced, to increase barrier ofvitriform transformation. On the other hand, if B is used at amountsmore than 5% by weight, other intermetallic compounts than Fe₂ B, suchas FeB, CrB and Cr₃ b₄, may be formed, which have a tendency to bestable crystalline structure in addition to preventing formation ofhomogeneous solid solution at just below melting point.

Mn has an effect of widening the partial band upon generation ofstacking fault by lowering the stacking fault energy. In turn, thiseffect increases dislocation density in the metastable crystallinephase, to promote the differentiation of dislocation with energyprovided from an external pressure. Within the alloy occurs a largestrain energy that is helpful for stable vitriform phase. Also, Mnserves as an important austenizing element in steel, transforminginstable austenite structure into martensite through working. Mn hashigh work hardenability and shows erosion resistance, when receivingrepetitive impulsive working attributed to cavity, in a wet atmosphere.By virtue of the erosion resistance, electrical corrosion and pittingcan be prevented, thereby showing high wear and corrosion resistancewhile maintaining a low friction coefficient. These effects of Mn cannotbe expected at an amount of less than 0.5% by weight. If Mn is used atan amount more than 3.2% by weight, gamma-Fe, i.e. austenite appears,inversely affecting the restraint of vitriform phase.

Si reinforces alpha-ferrite and improves the base strength when beingvitrified. Si is helpful for vitrification at an amount of more than0.2% by weight while such compounds as Fe₃ Si and Fe₂ Si start to appearat the amount of Si more than 1% by weight.

Since P is one of the vitrifying elements, it can be added but itsamount in Fe system is confined to no more than 0.5% by weight or lessin order to prevent brittleness.

C is a strong element to reinforce Fe but its amount is confined into arange of 0.05-0.3% by weight in order to prevent brittleness and toconstrain the formation of carbides. A trace of C reacts with alloyingelements, such as V and Nb, to form extremely fine carbide which notonly contributes to fineness of the particle size distribution inmetastable phase, to improve toughness, but serves to strengthen hightemperature strength.

Mo plays a role in forming and reinforcing α-ferrite. In addition, itexhibits an effect of autogenous alloy. Mo is added up to 2.5% by weightbecause excess amount does not have any additional effect. Mo, which hasa great influence on high temperature, is added at at least 0.3% byweight. Preferably, it is added at an amount of 2% by weight in view ofworkability and toughness.

As mentioned previously, F and Nb react with C to precipitate finecarbides which are helpful to improve the high temperature strength andto prevent the growth of particle size in the metastable phase.Particularly, V contributes to the improvement in high temperature creepstrength. When the sum of V and Nb is above 0.4% by weight, coarsecarbides occur, deleteriously affecting toughness and mechanicalproperties. Basic addition effect of this alloying metal can be achievedat at least 0.05% by weight.

A better understanding of the present invention may be obtained in lightof following examples which are set forth to illustrate, and are not tobe construed to limit, the present invention.

Alloy materials that had compositions indicated in Table 1 below weremade powders by atomization, which were then classified into an averageparticle size of 50 microns.

                                      TABLE 1                                     __________________________________________________________________________    Unit: wt %                                                                    Exam.                                                                             Cr  B  Mn  Si P   C  Mo  V  Nb  Fe                                        __________________________________________________________________________    A   26.7                                                                              3.42                                                                             1.06                                                                              0.52                                                                             0.01                                                                              0.15                                                                             0.8 0.1                                                                              0.05                                                                              bal.                                      B   27.4                                                                              4.1                                                                              1.84                                                                              0.30                                                                             0.02                                                                              0.21                                                                             1.2 0.15                                                                             0.07                                                                              bal.                                      C   27.2                                                                              3.86                                                                             1.44                                                                              0.31                                                                             0.01                                                                              0.15                                                                             1.6 0.11                                                                             0.06                                                                              bal.                                      __________________________________________________________________________

These uniform powders each were coated on S45C-based disks by plasmathermal spray and the resulting coating layers were polished until thethickness thereof was reduced into about 0.2 mm.

After S45C (AISI 1045) was subjected to quenching and then to tempering,induction hardening was effected on counter rings so that their contactmoving surfaces might have a hardness of HRC56-58.

In the examples, load carrying capacity test (LCCT) and endurance limittest (ELT) were carried out at P-250 kgf/cm2 and V-2.58 mm/min in theabsence of lubricating oil. For LCCT, the increment of load was 50 kgand each load was maintained for 1 min. Thereafter, ELT was followed. Inthis test, the times at which the coating layers were initiated intoabrasion were determined as change points of torque. The results areshown in Table 2 below. These tests are based on the present inventors'experiences that, in the presence of lubricating layer, abrasion scars,such as scoring and wipering, are hardly generated but, in the absenceof a lubricating layer, which may be caused under high surface pressure,such as local contact state, friction between metal and metal occurs,generating high temperatures in a moment, which damage the surfaces ofthe metals. Accordingly, the tests through which the maximum of thesurface pressure resistance and the duration time are determined in theabsence of lubricating layer are very important to assay the performanceof the surface.

In most cases, the coating layers have largely different hardnesses fromthose of the base structures. As hardness increases toughness decreases.In the case that the coating layer is thicker than 2 mm, the porosity inthe coating layer increases, which results in initiating break therein.Therefore, the thickness of the coating layer is confined up to 2 mm, inaccordance with the present invention. Also, the present inventors knewfrom various experiments that there exists a need of securing at least0.1 mm coating layer under a surface pressure of 200 kgf/cm². On thebasis of the experiences of the present inventors, the optimum thicknessof the coating layer is 0.2 mm, depending on the surface pressure andfriction rate.

                  TABLE 2                                                         ______________________________________                                                              Transfer                                                                             Final  Dynamic                                                 Initial by Serious                                                                           Temp. of                                                                             Frict. Weight                                           Abras.  Abras. Specimens                                                                            Coeffi.                                                                              Loss                               Exam. No.     (sec)   (sec)  (°C.)                                                                         (μ) (%)                                ______________________________________                                        A     A-1     1450    2950   85     0.106  0.009                                    A-2     1680    3260   88     0.097  0.011                              B     B-1     1518    3420   92     0.084  0.015                                    B-2     1620    3610   89     0.079  0.013                              C     C-1     1526    3340   86     0.086  0.012                                    C-2     1598    3460   88     0.89   0.010                              ______________________________________                                         note:                                                                         Hardness in plasma thermal spray state: HV (500 g) 480-660                    Outermost surface hardness after polishing: HV (10 g) 1250-1450               Thermal spray layer thickness: 0.18-0.22 mm                                   Hardness of base material: HV (500 g) 230-270                            

As apparent from Table 2, the present examples A, B and C all show gooddynamic friction coefficients not more than 0.15.

                  TABLE 3                                                         ______________________________________                                                            Dynamic                                                   Specimen Treatment  Friction                                                  No.  Ring        Disk       Coefficient                                                                            Remark                                   ______________________________________                                        1    HF hardening                                                                              HF hardening                                                                             0.72     Comparative                              2    HF hardening                                                                              PTFE coating                                                                             0.07*    Examples                                 3    HF hardening                                                                              Cu alloy + 0.13-0.23                                                          Graphite insert                                              ______________________________________                                         note:                                                                         *test not determined, low friction coefficient but extreme weight loss        owing to lack of strength.                                               

From the tables, it is apparent that the friction-worn portions can beremarkably improved in corrosion and wear resistance by coating the ironalloy of the present invention thereon.

Other features, advantages and embodiments of the present inventiondisclosed herein will be readily apparent to those exercising ordinaryskill after reading the foregoing disclosures. In this regard, whilespecific embodiments of the invention have been described inconsiderable detail, variations and modifications of these embodimentscan be effected without departing from the spirit and scope of theinvention as described and claimed.

What is claimed is:
 1. An erosion and wear resistant iron alloy,comprising: Cr 20.0-36.0% by weight, B 2.5-5.0% by weight, Mn 0.5-3.2%by a weight, Si 0.05-0.6% by weight, Mo 0.3-2.5% by weight, V 0.05-0.3%by weight, Nb 0.03-0.3% by weight, P 0.5% by weight or less, C 0.05-0.3%by weight, and a trace of unavoidable impurities.
 2. An erosion and wearresistant iron alloy in accordance with claim 1, comprising Cr26.0-28.0% by weight, B 3.2-3.6% by weight, Mn 0.3-1.0% by weight, Si0.2-0.5% by weight, V 0.1-0.3% by weight, Nb 0.05-0.2% by weight and thesum of P and C 0.5% or less.